Communication systems, such as Code Division Multiple Access (CDMA) systems, communicate messages between infrastructure equipment and mobile units. As used herein, a forward message refers to a message generated by cellular infrastructure equipment and transmitted for reception by a mobile communication unit, and a reverse message refers to a message generated by a mobile communication unit, such as a mobile cellular phone.
Power control is an important function in most communication systems and is essential to the proper operation of second generation (2G) and third generation (3G) cellular systems employing DS-CDMA. Power control is used to maintain sufficient communication link quality and information throughput while using minimum power levels to maximum system capacity. Changing the power levels of the forward communication links used by a base transceiver station (BTS) to transmit information to mobile stations is referred to as forward power control. Changing the power levels of the reverse communication links used by the mobile station to transmit information to serving BTSs is referred to as reverse power control.
Forward power control for second generation CDMA systems, such as IS-95 or J-ANSI-STD008 systems, typically use forward link quality information feedback transmitted on reverse links to update forward link power levels. This feedback information can be in the form of a bit used to indicate whether an individual forward link traffic channel (TCH) frame was erased. One such example is an erasure indicator bit (EIB) used in J-ANSI-STD008 systems. Another such example is a message to indicate some number of forward link frames were erased in a given time period, such as Power Measurement Report Message (PMRM) used in IS-95 or J-ANSI-STD008 systems.
Forward power control for third generation CDMA systems, such as proposed for CDMAOne, UMTS, and ARIB systems, typically use gain update information fed back on reverse links every 1.25 milliseconds or less and is referred to as fast forward power control. This feedback information can be punctured or sent on reverse link control channels.
As used herein, the term punctured refers to power control information bits replacing information bits in reverse link traffic channel frames. The gain update information is typically determined at the mobile station by comparing a measured signal-to-interference ratio to a signal-to-interference threshold determined by forward link quality. At the serving BTSs, the gain update information is used to decide whether to increase or decrease forward link gains resulting in a increase or decrease link forward link transmitted power.
In a typical CDMA system, when entering the boundary region between base transceiver stations, a mobile station transitions from communicating with one base transceiver station to communicating with both base transceiver stations simultaneously. Thus, the communication is not interrupted and the speech quality is not degraded, even temporarily. This multiple link communication is commonly referred to as soft-handoff. Significant diversity benefit is derived by the mobile station due to the receiving and combining of multiple signal paths from the serving base stations during soft-handoff, thereby allowing a net drop in forward link power. Soft-handoff can involve two or more BTSs.
For 3G systems utilizing fast forward power control involving soft-handoff, it is necessary to correct the forward TCH or forward link gains used at each BTS to account for gain update errors that occur over the reverse link channels. Typically while in soft-handoff, one or more of the reverse links will have a very high erasure rate or exhibit qualities that inhibit detection of the power control information, such as gain updates, for prolonged periods of time. As used herein, erasure rate refers to the percentage of frames received by a mobile unit from a BTS that are received with detectable bit or symbol errors, or that fail forward error detection/correction, or those frames whose transmission rate cannot be determined.
A 3G system employing fast forward power control corrects for high erasure rates on weaker reverse links relative to the dominant reverse link by employing a post-selection forward power control function. The post-selection forward power control function computes a new gain level based on information corresponding to the reverse link with the best quality as determined by the post-selection function. A new gain level is periodically sent to each serving BTS to update and synchronize the TCH gains.
One problem associated with such a post-selection gain correction method is when all reverse links indicate a high gain update error rate based on either all frames containing the gain update information being erased or all reverse link channels carrying the gain update information indicating a high error rate. In this case, if the power control update information transmitted every 1.25 milliseconds is used, it is possible to incorrectly power up or power down the forward links to the mobile station.
Spuriously powering up can create excess interference in the system. Powering down the BTS can degrade voice quality or cause a dropped call, depending on the duration of the reverse link outage. This problem can be especially severe in 3G systems because of the use of high power, high data rate supplemental forward link channels used along with a lower rate lower power fundamental channels. Typically the power control information corresponds to the fundamental channel.
Second generations systems similarly solve the poor reverse feedback link problem during soft-handoff by obtaining the forward link quality information, such as erasure indicator bits (EIBs) or Power Measurement Report Messages (PMRMs), used for forward power control from the best quality reverse link determined by the post-selection function. As used herein, the term dominant reverse leg refers to the link with the fewest reverse link frame erasures or whose frames have the lowest bit or symbol error rate or best decoder quality metric over a given time period.
In addition to poor coverage, the reverse links may also be all bad when a mobile station turns off its transmitter on the reception of a predetermined number of forward link bad frames. This method is employed in IS-95A, IS-95B and J-ANSI-STD008, where the predetermined number of forward link bad frames is set to twelve.
Consequently, a need exists for a method for performing forward power control when the feedback links are temporarily missing or the power control information received on the feedback links cannot be determined.